Nucleolin [8] is an abundant, non-ribosomal protein of the nucleolus, the site of ribosomal gene transcription and packaging of pre-ribosomal RNA. This 710 amino acid phosphoprotein has a multi-domain structure consisting of a histone-like N-terminus, a central domain containing four RNA recognition motifs and a glycine/arginine-rich C-terminus, and has an apparent molecular weight of 110 kD. While nucleolin is found in every nucleated cell, the expression of nucleolin on the cell surface has been correlated with the presence and aggressiveness of neoplastic cells [3].
The correlation of the presence of cell surface nucleolin with neoplastic cells has been used for methods of determining the neoplastic state of cells by detecting the presence of nucleolin on the plasma membranes [3]. This observation has also provided new cancer treatment strategies based on administering compounds that specifically targets nucleolin [4].
Nucleic acid aptamers are short synthetic oligonucleotides that fold into unique three-dimensional structures that can be recognized by specific target proteins. Thus, their targeting mechanism is similar to monoclonal antibodies, but they may have substantial advantages over these, including more rapid clearance in vivo, better tumor penetration, non-immunogenicity, and easier synthesis and storage.
Guanosine-rich oligonucleotides (GROs) designed for triple helix formation are known for binding to nucleolin [5]. This ability to bind nucleolin has been suggested to cause their unexpected ability to effect antiproliferation of cultured prostate carcinoma cells [6]. The antiproliferative effects are not consistent with a triplex-mediated or an antisense mechanism, and it is apparent that GROs inhibit proliferation by an alternative mode of action. It has been surmised that GROs, which display the propensity to form higher order structures containing G-quartets, work by an aptamer mechanism that entails binding to nucleolin due to a shape-specific recognition of the GRO structure; the binding to cell surface nucleolin then induces apoptosis. The antiproliferative effects of GROs have been demonstrated in cell lines derived from prostate (DU145), breast (MDA-MB-231, MCF-7), or cervical (HeLa) carcinomas and correlates with the ability of GROs to bind cell surface nucleolin [6].
AS1411, a GRO nucleolin-binding DNA aptamer that has antiproliferative activity against cancer cells with little effect on non-malignant cells, was previously developed. AS1411 uptake appears to occur by macropinocytosis in cancer cells, but by a nonmacropinocytic pathway in nonmalignant cells, resulting in the selective killing of cancer cells, without affecting the viability of nonmalignant cells [9]. AS1411 was the first anticancer aptamer tested in humans and results from clinical trials of AS1411 (including Phase II studies in patients with renal cell carcinoma or acute myeloid leukemia) indicate promising clinical activity with no evidence of serious side effects. Despite the promising clinical results from Phase II studies, AS1411 did not perform as expected in Phase IIB studies, possibly due to the low potency of AS1411.